Radiation-curable printing inks and coatings are increasingly becoming more popular. Water-based and solvent-based inks and coatings often require application of heat to dry. This limits the types of substrates that can be used. In addition, solvent-based inks often have undesirable properties such as unpleasant odors due to the volatility of the solvents.
The printing of flexible substrates is a rapidly growing market. Flexible substrates are useful for labels, and flexible films and foils for various packages. These packages may include packaging for food or pharmaceuticals, for example. Inkjet printing offers several advantages over other types of printing when using flexible substrates.
The market for the UV-inkjet printing of flexible substrates is now growing rapidly, especially for the printing of labels. An area which is now being addressed is the printing of labels and flexible films and foils for the food packaging sector. The use of single pass printing is becoming more common. Single pass printing can be defined as where the printhead array remains in a fixed position and the image printing is completed in a single pass while the material to be printed is passed underneath, or in front of, the printhead array. Single pass inkjet printing lends itself to high throughput printing applications such as the printing of reel-fed labels, flexible packaging, and the rapid printing of 3-dimensional objects such as plastic bottles.
A market area likely to develop and expand rapidly in the future is the single pass direct printing of rigid packaging, such as poly(ethylene) terephthalate (PET) drink bottles. At the moment, the rigid packaging (bottle) sector is mostly met by the application of preprinted labels and sleeves. The direct digital inkjet printing of bottles will thus remove the need for preprinted labels, leading to material savings and new design opportunities for this market. It is likely that for digital printing, inkjet will better lend itself to this market sector than other digital technologies, such as liquid toners (e.g. HP's ‘Indigo’ ‘Electroink’ process) and Landa's Nanography process, which both rely on an offset process whereby the image is applied to a heated offset roller prior to application to the substrate. These printing technologies require that the substrate pass between an impression and a blanket roller carrying the reverse print image.
In single pass applications, a UV-LED lamp source may be used immediately after a printing station to ‘pin’ the ink prior to any further printing operations. Further UV-curing at the end of the single pass operation can be used to deliver the desired full cure of the printed ink layer. ‘Pinning’ of an ink can be defined as where relatively low intensity UV-LED lamps are used to partially, or more fully, cure the inks very soon after application, thereby reducing the risk of droplets spreading on the printed substrate. Spreading of ink droplets on the substrate could lead to undesirable dot gain and result in loss of print quality.
The UV radiation necessary for curing can be provided from several sources, including mercury arc lamps and UV-light emitting diodes (UV-LED). Because of the lower consumption of energy, and reduced heating of the substrate, UV-LED curing is often preferable to use of other sources such as mercury arc lamps. The UV-LED bulbs do not generate ozone, in contrast to the typical UV-bulbs, require less energy, and exhibit a longer lifetime. Moreover, an additional advantage for UV-LED systems over the currently used UV-bulbs and UV-energy saving bulbs is the absence of mercury in the bulb. Therefore, UV-LED's are regarded as a “green” solution over typical mercury containing UV-bulbs.
However, despite the attractiveness of UV-LED curing, there are difficulties associated with using the low energy UV-LED curing. For example, it is often difficult to obtain good adhesion to the substrate. This may be due to inefficient curing of the ink or coating. Inks and coatings are often formulated in an effort to overcome problems such as poor adhesion to the substrate. For example, inks and coatings have been formulated which contain high concentrations of monofunctional monomers. It is known that monofunctional monomers promote good adhesion to substrates, including flexible substrates. However, the inclusion of such high concentrations of monofunctional monomers would most probably make them unsuitable for those applications requiring low migration from the cured inks. Migration can be defined as the diffusion of low molecular weight components from a printed ink to cause contamination issues. Where the printed substrate forms part of a package for food, for example, these low molecular weight components could diffuse into the foodstuff itself, causing undesirable and potentially health threatening contamination of the food. The use of high concentrations of monofunctional monomers in a UV-curable inkjet product is a common practice to achieve the required adhesion, but such monomers are less likely to be reacted into the UV-cured ink network compared with multifunctional monomers, and thus substantial quantities of uncured monofunctional monomer will remain in the cured ink and be free to migrate from the ink, causing undesired contamination.
Other methods of improving adhesion that have been tried are, for example, including various additives such as surfactants in the formulations, and applying primer layers to the substrate to be printed. The use of primers, although a technically feasible approach to achieving improved adhesion of UV-cured inkjet inks to plastic surfaces, is undesirable since it introduces a further step into any printing operation.
U.S. Pat. No. 8,759,412 describes how polymerizable photoinitiators containing thioxanthone moieties can be used for the UV-LED curing of coatings and inks. The polymerizable groups on the photoinitiator will react into the ink, thereby reducing its likelihood of migrating from a cured coating/ink film. WO2014032968 describes the incorporation of thiols into a UV-curable ink to improve its cure response. However, compositions comprising thiols often have poor long-term stability, which is likely due in part to the capacity of thiols to react with acrylates via the ‘Michael’ reaction. JP2009249562 uses the Michael reaction ability of polyfunctional thiols to form dendritic polymers with molar excesses of multifunctional acrylate monomers. The products of such reactions enable high adhesion strength on PET (poly(ethylene terephthalate)).
EP2671722 describes UV-curable inkjet products comprising greater than 5 wt % of a (meth)acrylated silicone surfactant to deliver improved adhesion to plastic substrates when tested using the tape adhesion test. It is quite likely that some of the observed improvement achieved with the inclusion of such reactive silicones was a result of the reduced adhesive strength between the adhesive tape and the ink surface, resulting in a reduced force applied through the ink layer to the underlying substrate.
WO2002/085638 and US2012/0058317 describe the use of primer layers applied to the substrate prior to printing to achieve improved adhesion. In WO2002/085638 solvent-based or water-based primers comprising acrylic or polyurethane resins are disclosed. US2012/0058317 discloses a UV-curable primer containing a polyester acrylate along with a high concentration of an acrylated amine.
Including low molecular weight acrylic resins into UV-curable inkjet inks has been described, but generally these formulations require the incorporation of significant quantities of monofunctional monomers, such as vinyl caprolactam, phenoxyethyl acrylate, cyclic trimethylolpropane (TMP) formal acrylate and isobornyl acrylate. As described above, a high concentration of monofunctional monomers can lead to problems with migration, particularly with flexible packaging. WO2008/004002 (Sericol) demonstrates inkjet inks based solely on monofunctional monomers and does not show how adhesion can be achieved with inks based solely or largely on multifunctional monomers. WO2008/004002 describes that the inks should be substantially free of multifunctional monomers, where preferably less than 15 wt % and most preferably less than 2 wt % of the total ink composition comprises multifunctional monomers. In addition, this patent states that it is most preferable that the passive thermoplastic resin (the inert acrylic resin) should form less than 8 wt % of the total ink composition. Furthermore, none of the examples show how to achieve concentrations of more than 10 wt % of an inert acrylic resin in the ink composition. EP2666832 (HP) clearly states the need to include a monofunctional monomer to achieve the desired adhesion to plastic surfaces. Indeed, the comparative examples of this patent show that inks based solely on multifunctional monomers have both poor adhesion and too high viscosity. U.S. Pat. No. 7,662,224 (D. D. Sloan) describes the inclusion of inert acrylic resins into UV-curable inkjet fluids, although the inks also contain an organic solvent diluent.
There remains a need to develop inks and coatings suitable for inkjet printing that can be cured using UV-LED light. Preferably, this would be done without the use of components that are likely to migrate from the cured ink or coating and contaminate, for example, food or pharmaceuticals in packaging on which the ink or coating is applied.